A horizon

soil type

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Soil profiles on hillslopes

The thickness and composition of soil horizons vary with position on a hillslope and with water drainage. For example, on the upper slopes of poorly drained profiles, underlying rock may be exposed by surface erosion, and nutrient-rich soils (A horizon) may accumulate at the toeslope. On the other hand, in well-drained profiles under forest cover, the leached layers (E horizon) may be relatively thick and surface erosion minimal.

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composition

…surface litter) is termed the A horizon. This is a weathered layer that contains an accumulation of humus (decomposed, dark-coloured, carbon-rich matter) and microbial biomass that is mixed with small-grained minerals to form aggregate structures.

passage of time

…the humus content of the A horizon, which, for soils less than 10,000 years old, increases continually at a rate dependent on parent material, vegetation, and climate. Typically, this rate of increase slows after about 10,000 years, plant nutrients begin to leach away, and a significant decline in humus content…